18W passive pfc led lighting driver design-EEWORLD

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1. Design Features

1. Very high efficiency: 82%

2. Small number of components : only 40 components are needed 3. No common mode inductor is required to meet the EN55022B requirements for conducted EMI 4. Valley filling circuit enables the power supply to meet IEC61000-3-2 THD limits 5. ON/OFF control suppresses the high power frequency ripple voltage caused by the valley filling (THD correction) circuit

2. Circuit Schematic Diagram

Figure 1. LED lamp with passive PFC Driver Power supply schematic

3. Working Method

The flyback converter shown in Figure 1 uses a device from the TinySwitch-III series (U2, TNY279PN) to provide up to 1.8 A load current to 6 high- brightness lumen LEDs (LXHL series).

The output voltage is slightly lower than the forward voltage drop of the LED string. Therefore, when the LED string is connected to the power supply, the power supply operates in constant current (CC) mode. If the LED string is not connected to the power supply, the voltage regulator VR1 provides voltage feedback to regulate the output voltage to about 13.5 VDC. A 100 mW resistor (R11) senses the output current and drives an optocoupler through an op amp (U1) to provide feedback to U2. The TinySwitch-III series devices regulate by shutting down or skipping MOSFET switching cycles. When the load current reaches the current setting threshold, U1 drives U3 to conduct. The phototransistor in U3 pulls current from the EN/UV pin of U2, causing U2 to skip cycles. Once the output current drops below the current setting threshold, U1 stops driving U3, U3 stops pulling current from the EN/UV pin of U2, and switching cycles are re-enabled. The TL431 (U4) provides a reference voltage to U1 for comparison with the voltage drop across R11.

The output rectifier (D9) is located at the lower leg of the transformer (T1) secondary winding to reduce EMI noise generation. The RCD clamp (R16, C4 and D13) protects the MOSFET drain from flyback voltage spikes.

The valley filling circuit (D5, D6, D7, C15, C16 and R15) limits the 3rd and 5th harmonic values of the power frequency current so that the power supply meets the total harmonic distortion (THD) requirements specified in IEC61000-3-2.

The frequency jittering function of U2, the shield winding in T1 and the Y capacitor (C8) across T1 work together to reduce the generation of conducted EMI, so a simple pi-type filter (C13, L1, L2 and C14) can make the power supply meet the limits of EN55022B.

4. Design points

1. Take the input capacitance value calculated by PI Expert or PI Xls, divide it by 2, and round it to the next larger standard value to select C15 and C16.

2. Use PI Expert or PI Xls to consider the maximum output power of the power supply at the maximum VF of the LED . 3. There are two op amps in LM358 (U1). Make sure the input of the second op amp (pins 5 and 6) is connected to the secondary ground.